1. Green, W. D. US Patent 1773 997 1930
2. Klepetko, E., Bauer Kaye, P. D., Hartzell, L. J. and Tooele, Jr. US Patent 2 506 301 1950
3. Klepetko, E., Bauer Kaye, P. D., Hartzell, L. J. and Tooele, Jr. US Patent 2 591 830 1952
4. paper presented at 119th Annual Meeting;Irwin,1990